This invention relates-to a transporting apparatus for scanning exposure that is suitable for use with an image reading and/or recording apparatus, that adopts a screw transmission mechanism and that causes only limited erratic transport of a reading and/or recording material.
Optical scanning such as raster scanning and slit scanning is applied to various types of image recording apparatus. In image recording apparatus of a type that adopts raster scanning, a recording light beam is deflected one-dimensionally in the direction of main scanning while a recording material is transported for sub-scanning in a direction generally perpendicular to the direction of main scanning and, as a result, the light beam is scanned two-dimensionally over the entire surface of the recording material, thereby recording an image on it. In image recording apparatus of a type that adopts slit scanning, a recording light source forming a slit is transported for scanning in a direction generally perpendicular to the length of the slit, or a recording material is transported for scanning relative to the light source in a slit form, whereby the recording light is scanned over the entire surface of the recording material to record an image.
With these apparatus that adopt raster scanning or slit scanning, it is well known to transport a large-size document or recording material by means of a transporting apparatus for light beam scanning in which an exposure mount for fixing an object to be scanned such as the document or recording material is moved by a screw transmission mechanism composed of a drive screw (hereunder referred to as "a screw shaft") and a travelling nut (hereunder referred to simply as "a nut"). See, for example, Japanese Utility Model Laid-Open Application (jikkai) No. 184367/1986, which proposes an image recording apparatus that performs transport in the sub-scanning direction using a transporting apparatus with a screw transmission mechanism and that uses a servo motor as a source for rotationally driving the screw shaft with a view to achieving even and correct transport in the sub-scanning direction.
In order to achieve satisfactory image recording without causing any unevenness in the quality of image, it is necessary that the recording material be transported for scanning at a constant speed in a correct and even manner. However, the aforementioned image recording apparatus which simply uses a servo motor suffers from the disadvantage that external forces, such as those applied by vibrations due to erratic rotation of the motor or eccentricity of the screw shaft, as well as those applied by impacts, and those applied by vibrations due to variations in the force of friction with guide members during the movement of the exposure mount or due to the seams of the guide members, cause the exposure mount to vibrate, leading to an erratic transport of the recording material.
As already mentioned, the exposure mount (or the recording material to be scanned) must be transported at a predetermined constant speed in a correct manner in order to accomplish satisfactory image recording. However, if unevenness occurs in the transport speed on account of the factors described above, the recorded image will have unevenness in color and density in the sub-scanning direction.
Unevenness in the sub-scanning speed (i.e., the speed of transport of the recording material to be scanned) bears particular importance in image recording apparatus that adopt raster scanning because unevenness occurs in the pitch of main scanning lines, producing an image in which noticeable unevenness appears as stripes in the sub-scanning direction.
The transporting apparatus for scanning exposure that transports the recording material by moving the exposure mount with the aid of a screw transmission mechanism has not yet solved the aforementioned problems of erratic transport due to erratic rotation of the motor or eccentricity of the screw shaft, or erratic transport due to external vibrations or forces such as those exerted by the exposure mount when it rides over the seams of guide members or variations in the transport load due to changes in the force of friction between the guide members and the exposure mount. This has presented particular difficulties in applying the transporting apparatus to image recording apparatus that adopt raster scanning.